1. Field of the Invention
This invention pertains to the general field of packages and packaging methods for semiconductor devices; in particular, the invention relates to an improved plastic encapsulation apparatus and method for a very thin device having an integrated circuit bonded to a lead frame and packaged in molten plastic according to procedures commonly used in the industry.
2. Description of the Prior Art
In a typical prior art system for bonding and encapsulating integrated circuits, a plurality of such circuits is arranged in a work-piece that contains a series of identical lead frames etched or stamped on a strip of metallic substrate. A chip is bonded to each of these premanufactured lead frames and they are loaded into a bottom mold having a cavity and shallow impressions corresponding to each lead frame, the number of lead frames in a single batch being determined by the mold size and the capacity of the particular molding equipment used. A top mold having cavities aligned with the cavities in the bottom mold is moved into place on top of the bottom mold, so that the two molds together define a cavity corresponding to the intended volume of each integrated circuit package. Some means is provided within the molding equipment to heat the molds to the proper temperature for molding. The heated molds are clamped together and, when the temperature is right, usually at approximately 175.degree. C., molding material, such as epoxy in pellet form, is placed in the mold set through bores formed in the top or bottom mold. Plungers are then inserted into the bores and a pressurizing force is exerted on the plungers. The combination of the plunger force and the temperature causes the epoxy pellets to liquify and flow into the cavities provided in the mold set, which determine the configuration and location of the molded plastic that encapsulates the integrated circuits.
Alternatively, in some equipment molten plastic is forced into the cavity defined by the top and bottom molds through a flow conduit connected either to the top or bottom mold. A pressure source forces the molten plastic from a retaining reservoir, where it is heated to flow conditions, through the conduit, into the top or bottom cavity, and around the integrated circuit and lead frame to form a package for the device. The plastic flows from one side of the lead frame to the other, filling the cavity and thereby encapsulating the integrated circuit and adjacent portions of the lead frame contained therewithin, as determined by the geometry of the cavity.
All apparatus and methods of the prior art only inject plastic into either the top or bottom cavity, depending on pressure to force the plastic to the other cavity, around the integrated circuit and lead frame. This approach can cause air pockets to be formed in the plastic material flowing under the lead frame. In addition, it requires high operating pressures to force the plastic to flow from one portion of the mold cavity through the lead frame sandwiched between the top and bottom molds. A special problem exists when dealing with very thin devices that require particularly thin plastic packages (in the order of one millimeter). In such cases the presence of air pockets in the plastic is even more critical to the life and performance of the device. Therefore, a special need exists to ensure that all air is removed during the molding process.
Accordingly, there exists a need to provide an improved plastic encapsulation apparatus which not only provides a more uniform flow of plastic around the lead frame, as achieved by the invention disclosed in my copending application referenced above, but that also forces trapped air to flow out of the molten plastic, thereby reducing the likelihood of air pockets and other imperfections remaining in the finished plastic package. This is the objective to which this application is directed.